The invention relates to a system and method for controlling reductant injection upstream of a selective reduction catalyst for use with an internal combustion engine.
In order to meet emission regulations, selective catalytic reduction (SCR) systems using externally added reducing agents are used. In such a system, regulated emissions, such as certain nitrogen oxides, or NOX, can be reduced in an oxygen-rich environment to nitrogen over a catalyst when a reducing agent or reductant, such as urea water solution (urea) or ammonia (NH3), is added.
Current SCR systems are very effective in NOX reduction at temperatures above 250xc2x0 C. and have potential to substantially reduce NOX emission from diesel vehicles. However, the NOX reduction efficiency of SCR systems using urea or ammonia as reductant is rather low at and below 200xc2x0 C., which limits its overall NOX reduction efficiency for many diesel operation conditions.
Various systems for controlling ammonia injection are proposed in the prior art. These prior art systems inject urea in proportion to the NOX concentration level above certain temperatures. One approach to increase the SCR NOX reduction efficiency is to inject excess amount of reductants at low temperature. However, tests show that the addition of excess ammonia or urea increases the NOX reduction efficiency only slightly and may cause a large ammonia emission.
We have discovered that urea injected into a SCR catalyst is much more effective, if it is injected when the engine-out NOX concentration is below a threshold value, which is in the range of 25-75 ppm. The main effect is to significantly enhance the reductant storage on SCR catalysts under these conditions. The storage of reductant is only partially effective when the engine-out NOX is above that level. High NOX levels inhibit the adsorption of the reductant on active sites adjacent to NOX to promote NOX reduction especially at temperatures below 250xc2x0 C.
The present invention proposes a new method of enhancing the SCR NOX reduction efficiency at low temperature. It involves injecting relatively larger amounts of reductant into the catalyst while the engine produces low NOX emission during idling or deceleration. This allows the reductant to be stored in the SCR catalyst before the catalyst is exposed to the high concentration of NOX. The reductant injection is stopped when the catalyst storage capacity is nearly full. The resultant NOX reduction is much greater than without such storage. For example, excess urea was stored in SCR catalyst in engine deceleration condition, and upon subsequent acceleration, a largely enhanced NOX reduction efficiency was observed at low temperature. No reductant was injected into the system during the acceleration, so the only reductant available for NOX reduction was the urea (ammonia) pre-stored in the SCR catalyst. The NOX conversion at 200xc2x0 C. was 90% compared to 60% for a similar test without such storage. The SCR NOX conversion light off (50% conversion) temperature was reduced about 60xc2x0 C. from 210xc2x0 C. to 150xc2x0 C. The method of the present invention can greatly improve the SCR NOX reduction efficiency at low temperature for diesel vehicle applications.
In practice, a method of injecting urea when engine-out NOX is low may sometimes inject urea at the same point as a method of injecting urea when the catalyst temperature is low. The engine-out NOX level may be low at idle, for example. After running at idle for some time, the catalyst temperature may also be low. Therefore, both strategies would inject at idle. However, the strategies are different on decelerations. The method of the present invention being based on engine-out NOX would begin urea injection immediately on deceleration because the engine-out NOX level drops quickly. However, a method based on the catalyst temperature would not begin urea injection until the catalyst cooled down, which could take 20 to 60 seconds or perhaps never happen if the driver accelerates. In addition, the two strategies will respond differently to high EGR levels, which may provide low engine-out NOX levels while maintaining high catalyst temperatures. The method of the present invention provides more reductant to the active sites in the SCR catalyst to overcome NOX inhibition, and this increases NOX reduction efficiency.
In accordance with the present invention, a system and method for controlling ammonia injection upstream of a selective reduction catalyst (SCR) is disclosed that overcomes NOX inhibiting effects by injecting excess reductant into the SCR catalyst when engine-out NOX concentration is below a preset value such as when the engine is idling or decelerating. During these operations, the NOX adsorbed at the active sites is purged out by the low NOX exhaust flow, and the NOX inhibiting effects is minimized, thus the reductant can freely access the active sites to conduct a high NOX reduction process.